1. Field of Invention
The invention relates to optical color matrixing using time delay and integration sensors to perform a color matrix operation that provides an improvement in color quality.
2. Description of Related Art
In most color document scanners, some form of electro-optical imaging system is used to produce a set of color image signals. Often, there are three such signals that can generally be described as red, green and blue, RGB. These signals are xe2x80x9cdevice dependentxe2x80x9d because they depend on characteristics of the scanner.
If these signals are related to the human visual system in a certain way, the device dependent signals can be processed to produce a set of signals which are xe2x80x9cdevice independentxe2x80x9d. That is, the three color image signals produced by the scanner can be expressed by three color scanner coordinates RGB, to which a transformation can be applied to describe the three scanner coordinates in terms of three visual coordinates XYZ. As used herein, these visual coordinates are tristimulus values or similar coordinates related to tristimulus values. The visual coordinates can be transmitted to an output device such as a printer, copier or display screen to output an image that corresponds to the scanned image. Because the visual coordinates are device independent, they can be transmitted to different copiers, printers or display screens, and generally the same image will be output regardless of the characteristics of the scanner, copier, printer or display screen.
The ideal transformation between RGB and XYZ coordinates is a linear matrix operation shown in equations 1-3:
X=k1*R+k2*G+k3*Bxe2x80x83xe2x80x83Eq. 1
Y=k4*R+k5*G+k6*Bxe2x80x83xe2x80x83Eq. 2
Z=k7*R+k8*Gxe2x88x92k9*Bxe2x80x83xe2x80x83Eq. 3
A more compact expression is given by the matrix equation
v=Ks.xe2x80x83xe2x80x83Eq. 3A
where v and s are column vectors of the visual and scanner coordinates, respectively, and K is a transformation matrix made up of kn coefficients.
It is common to try to design the scanner so that these equations are at least approximately true. In addition, it is known to determine the kn coefficients by procedures in such a way that the equations are satisfied as close as possible. Optimization schemes have been designed to try to minimize the color errors of scanned inputs by appropriate selection of the coefficients kn in the transformation matrix K.
A time delay and integration (TDI) color scanner (FIG. 3) is an electronic color scanner that uses a sensor having N rows or stages of photosites or pixels replicated in each of M groups of the N rows. For example, TDI sensors are known in which 96 stages are provided (N=96) in three groups (M=3). Each group of N rows is covered with a color filter. When M=3, the color filters are often red, green and blue. It is possible to improve the performance of the scanner by increasing the number of groups and corresponding filters beyond three to correct for the fact that the filters are not ideal (i.e., they do not satisfy Equations 1-3). By adding filters that are independent of the red, green and blue filters, and by modifying the transformation equations accordingly, it is thought that a better approximation to the visual coordinates XYZ values can be obtained.
In the TDI scanner, the N rows of photosites are scanned synchronously to make the apparent dwell time of each spot on the input document N times longer than for a single row. In a TDI system, the total signal from a series of stages of single color groups is given by:
Sm=xcfx84xcexa3n=1 . . . N[HnmrmOmfmRnm]xe2x80x83xe2x80x83Eq. 4
where
xcfx84 is the integration time,
N is the number of stages,
Hnm is the m-color irradiance at the position corresponding to the nth stage,
rm is the m-color reflectance at the point of interest,
Om is the m-color throughput of the optical system,
fm is the m-color filter transmittance, and
Rnm is the responsivity of the m-color sensor in the nth stage.
If the illumination and responsivity are assumed to be constant over space and time, the signal is
Sm=Nxcfx84HmrmOmfmRmxe2x80x83xe2x80x83Eq. 5
For example, the xe2x80x9credxe2x80x9d signal R is given by the sum over all stages in the xe2x80x9credxe2x80x9d group as follows:
R=Nxcfx84HmrrOrfrRrxe2x80x83xe2x80x83Eq. 5A
In the known TDI scanner, a different color filter covers each group m of N rows. The effective spectral sensitivity of each group m of N rows can be improved if some of the photosites in one group are covered by filters from other groups.
To achieve this and other objects, the invention relates to a time delay and integration color scanner for producing a first set of device dependent color signals that may be transformed into a corresponding set of device independent color signals using a transformation matrix. The scanner includes photosites arranged in rows and divided into groups of the rows; and a color filter associated with each group. Each color filter covers more or less than the rows in its corresponding group and some or none of the rows in other groups. The color filters produce a second set of color signals that are linear combinations of the first set of device dependent color signals and are based on a number of rows of each group that are covered by each filter. The second set of color signals may be an approximation to the device independent color signals.